Processes of incinerating waste materials and controlled incinerators of the so-called "starved-air" type have been well developed over the past decade for incinerating a wide variety of waste material with extremely low particulate emission. These incinerators are often connected with waste heat recovery systems in the form of steam boilers or the like for utilizing the hot combustion gases for recovering heat produced during incineration. Most waste materials which would be incinerated can be basically described as being composed primarily of (1) volatile matter, (2) moisture, (3) fixed carbon and (4) non-combustibles (ash or inert residue).
In these controlled starved-air incinerators and processes, the waste is initially burned in a main combustion chamber through the use of an underfire system supplying air to support combustion at less-than-stoichiometric requirements or theoretical air required for complete combustion of the waste materials which results in very slow burning at low temperatures. This, in effect, acts like distillation whereby the volatile matter and moisture are vaporized and a portion of the fixed carbon is converted to vaporized volatile matter, all of which pass to a secondary combustion chamber. This slow burning at low temperatures in the main combustion chamber is necessary to reduce turbulence created during this initial burning to minimize non-combustible particles from passing with the vapors into the secondary combustion chamber and to prevent vaporization of some of the inorganic substances in the non-combustibles, which may result in stack opacity problems and particulate emission rates which are higher than permissible under many current state and federal environmental laws. Additionally, slow burning at low temperatures in the main combustion chamber is desirable for lessening clinker formation resulting from ash fusion and melting of certain inorganic materials, such as low melting point metals, glass, etc.
The vaporized volatile materials are thereafter burned in the secondary combustion chamber under greater-than-stoichiometric conditions, thereby effecting substantially complete combustion of such vapors and conversion to organic materials for emitting to the atmosphere. The non-combustible inert residue or ash is then removed from the main combustion chamber of the incinerator for disposal in landfills or the like.
Notwithstanding the attempts to effect burning in the main combustion chamber at low temperatures by an underfire air system which supplies air at less-than-stoichiometric requirements for the waste materials, most prior incinerators and processes have suffered from problems with localized high temperatures around the individual underfire air supply manifolds or pipes. In these localities, the air may be greater-than-stoichiometric with respect to waste materials in that immediate vicinity resulting in the above-described problems occurring of undesirable clinker formation and vaporization of non-combustibles which results in higher than desirable particulate emission from the incinerator.
An additional problem has been presented in such prior controlled starved-air incinerators and processes, in that, due to the very nature of less-than-stoichiometric burning in the main combustion chamber, a significant portion of the fixed carbon in the waste materials is not converted to volatile matter and exits the incinerator as partially burned char in the non-combustible inert residue or ash. This creates a problem in that many states will not accept such residue in their landfills if the residue has more than a certain level of residual combustibles therein. Many pathogenic wastes and other hazardous wastes must have maximum burning of the volatile materials therein. Additionally, the incomplete combustion of fixed carbon in the waste materials results in a loss of overall thermal efficiency of the incinerator which becomes of significant importance when the incinerator is mated with a waste heat recovery system.